This invention relates to an optical glass having a negative anomalous partial dispersion .DELTA.P.sub.g,F, an index of refraction of n.sub.d .gtoreq.1.67 and an Abbe number of .nu..sub.d &gt;36.
As a measurement for the dispersion of an optical glass between wavelengths F (.lambda.=486.13 nm) and C (.lambda.=656.27 nm) the Abbe number .nu..sub.d is used, which is determined according to the equation ##EQU1## in which n.sub.d, n.sub.F and n.sub.C represent the indices of refraction at wavelengths F, C and d (.lambda.=587.56 nm).
The relative partial dispersions are introduced for wavelength ranges outside of F and C. The relative partial dispersion P.sub.x,y, relative to wavelengths x and y of the visible light, is given by: ##EQU2##
Like the Abbe number, the relative partial dispersion is an important constant for an optical glass. The majority of optical glasses substantially satisfy the approximately linear relationship: EQU P.sub.x,y =a.sub.x,y +b.sub.x,y .nu..sub.d, (3)
which is established by two standard glasses, e.g., the two optical glasses F2 and K7 (SCHOTT-Katalog "Optisches Glas," Edition IX/80, section 2, pages 5-20). By combining glasses with different Abbe numbers, the chromatic aberration in the lens systems for two colors can be successfully eliminated. The remaining chromatic aberration continuing to exist for the other colors is designated as the secondary spectrum. But by the use of glasses with widely deviating partial dispersions, it is possible to reduce the secondary spectrum in lens systems correspondingly, thus substantially improving the correction of the lens system. For a number of years the producers of optical glasses have tried, in order to meet the requirements of manufacturers of optical equipment, to develop glasses that can be used to greatly reduce the secondary spectra of systems with the further objective of providing a correction in the blue region of the visible spectrum. In this region of the spectrum, the relative partial dispersion P.sub.g,F for the wavelengths g (.lambda.=486.13 nm) characterize the optical behavior of a glass.
In DE-OS 22 65 703, glasses are described having optical constants of 1.59 to 1.86 for n.sub.d and 62 to 19 for .nu..sub.d, which glasses contain 2-50% by weight of highly expensive Gd.sub.2 O.sub.3 and thus can be produced only at great cost. In DE-OS 22 62 364 glasses of a B.sub.2 O.sub.3 - PbO - Al.sub.2 O.sub.3 system are described, which because of the high B.sub.2 O.sub.3 content have a low chemical resistance, a high density and a low devitrification stability (strong tendency toward crystallization). In DE-OS 19 44 284 and JP-OS 53-90311 (1978) glasses from the same system are described, in which by addition of SiO.sub.2, an attempt was made to improve the chemical resistance, but this was satisfactory only with high SiO.sub.2 contents. Furthermore, the high devitrification tendency remained unsatisfactory.
Boron silicate glasses containing zirconium are known from DE-OS 27 29 706, which glasses must contain at least 13.5% by weight of ZrO.sub.2. It is known, however, that such high proportions of ZrO.sub.2 promote the instability of the glass. Moreover, a high alkali content is necessary to achieve good meltability for such a glass, but the high alkali content adversely affects chemical resistance. Also, the high ZrO.sub.2 proportion and the large amounts of the Ta.sub.2 O.sub.5 added in almost all examples of this specification result in a high price of this glass. Similarly high costs are involved in the glasses described in DE-OS 20 24 912, which contain at least 10% by weight of GeO.sub.2 and at least 20% by weight of Ta.sub.2 O.sub.5.
In U.S. Pat. Nos. 2,762,713 and 4,211,569, glasses are described which have little or no B.sub.2 O.sub.3, and part of the glasses contain high proportions of SiO.sub.2, both of which conditions promote devitrification.
The glasses according to U.S. Pat. No. 3,888,692 contain less than 20% by weight of B.sub.2 O.sub.3, at least 12.5% by weight of SiO.sub.2 and at least 8% by weight of Al.sub.2 O.sub.3. By exceeding this boric acid content, resistance to weathering decreases and devitrification occurs. The high SiO.sub.2 content requires high melting temperatures and prolonged melting times, by which the attack on the melting vessel walls is accelerated.
Thus, despite many compositions taught by the prior art, all of them have at least one undesired characteristic.